A consortium of consumer electronics manufacturers, among which Royal Philips Electronics, has been working on specifications for a core of API's (application programming interfaces) for digital consumer electronics appliances in a home network so as to provide a standard for the audio/video electronics and the multimedia industries. An API specifies the method required for making requests to an operating system or application program. The home network is considered a distributed computing platform. The primary goal of the standard, referred to as the HAVi (Home Audio/Video interoperability) architecture is to ensure that products of different vendors can interoperate, i.e., cooperate to perform application tasks. Current CE devices, such as home entertainment equipment (DVD players, DV camcorders, digital TV sets, etc.) are digital processing and digital storage systems. Connecting these devices in networks makes it possible to share processing and storage resources. This allows coordinating the control of several CE devices simultaneously, e.g., in order to simplify user-interaction. For example, a first device may instantiate recording on a second device while accessing an EPG (electronic program guide) on a third device. The home network provides the fabric for connecting the CE devices. It allows connected devices to exchange both control (one device sending a command to another) and AV (audio/video) data (one device sending an audio or video stream to another device). The network has to meet several requirements in order to achieve all this. It must support timely transfer of high-data-rate AV streams. The network must support self-configuration, self-management, and hot plug-and-play. It must require low-cost cabling and interfaces.
The HAVi software architecture is platform-independent and based on Java. HAVi uses the IEEE 1394 high-performance serial bus protocol for transport of control and content among the devices connected to the network. The IEEE 1394 standard is a dynamically configurable, low-cost digital network. IEEE 1394 defines both a backplane physical layer and a point-to-point cable-connected virtual bus implementations. The backplane version operates at 12.5, 25 or 50 Mbits/sec. The cable version supports data rates of 100, 200 and 400 Mbits/sec. The standard specifies the media, topology, and the protocol. The IEEE 1394 transport protocol is particularly useful for supporting audio and video communication protocols, due to its high data-rate capability.
The HAVi architecture controls the CE devices in the network through abstract representations of the CE devices. The abstract representations are operated upon by a controller and hide the idiosyncrasies of the associated real CE devices. The abstract representation thus provides a uniform interface for higher levels of software. The abstract representations are registered with their control properties reflecting those of the device represented. The abstract representations expose their Interoperability API's to the applications and collectively form a set of services for building portable, distributed applications on the home network.
The architecture allows a device to send a command or control information to another device in the home network. A HAVi-compliant device contains data (above abstract representation, referred to as Device Control Model or DCM, see further below) relating to its user-interface (e.g., GUI) and to its control capabilities. This data includes, for example, HAVi bytecode (Java) that can be uploaded and executed by other devices on the network. A HAVi-compliant device has, as a minimum, enough functionality to communicate with other devices in the system. During interaction, devices may exchange control and data in a peer-to-peer fashion. This ensures that at the communication level, none of the devices is required to act as the master or controller of the system. On the other hand, it allows a logical master or controller to impose a control structure on the basic peer-to-peer communication model. HAVi distinguishes between controllers and controlled devices as explained further below. A controller is a device that acts as a host for a controlled device. A controller hosts the abstract representation for the controlled device. The control interface is exposed via the API of the abstract representation. This API is the access point for applications to control the device.
HAVi-compliant CE devices are devices categorized as follows: Full-AV devices (FAV's), Intermediate-AV devices (IAV's) and Base-AV devices (BAV's).
An FAV contains a complete set of the software components of the HAVi-software architecture (see below). An FAV is characterized in that it has a runtime environment for HAVi bytecode. This enables an FAV to upload bytecode from other devices for, e.g., providing enhanced capabilities for their control. An FAV may be formed by, e.g., a HAVi-compliant Set Top box, a HAVi-compliant Digital TV receiver, and an home PC. For example, an intelligent TV receiver can be the HAVi controller of other devices connected on the network. The receiver gets the bytecode uploaded from another device for creating a UI for this device and for providing external control of this device. An icon presenting this device can be made to appear on the TV screen and user interaction with the icon may cause elements of the control program to actuate the represented device in a pre-specified manner.
An IAV does not provide a runtime environment for HAVi bytecode, but may provide native support for control of specific devices on the home network. An IAV comprises embedded software elements that provide an interface for controlling general functions of the specific devices. These software elements need not be HAVi bytecode and may be implemented as native applications on the IAV that use native interfaces to access other devices.
A BAV may provide uploadable HAVi bytecode but does not host any of the software elements of the HAVi architecture. A BAV is controllable through an FAV by means of the former's uploaded bytecode. A BAV is controllable through an IAV via the native code. Communication between an FAV or IAV, on the one hand, and a BAV on the other hand requires that the HAVi bytecode be translated to and from the command protocol used by the BAV.
The main software elements included in the core specification of the HAVi architecture are the ones listed below. For a more detailed explanation of these elements, please see the HAVi spec., herein incorporated by reference.
1) A 1394 Communications Media Manager (CMM)--acts as an interface between the other software elements and the IEEE 1394. PA1 2) An Event Manager (EM)--informs the various software elements of events in the network such as the changes in the network configuration that occur when appliances (devices) are added or removed from the network. PA1 3) A Registry--maintains information about the appliances connected to the network and the functions they offer. Applications can obtain this information from the registry. PA1 4) A Messaging System (MS)--serves as an API that facilitates communication between the software elements of the various appliances on the network. The messaging system provides the HAVi software elements with communication facilities. It is independent of the network and the transport layers. A messaging system is embedded in any FAV and IAV. The messaging system is in charge of allocating identifiers for the abstract representations at the FAV or IAV. These identifiers are first used by the abstract representations to register at the FAV or IAV. Then they are used by the abstract representations to identify each other within the home network. When a first abstract representation wants to send a message to another abstract representation it has to use the identifier of the latter while invoking the messaging API. PA1 5) A Device Control Module (DCM)--represents an appliance on the network. Application programs can interact directly with a DCM. This shields them from the idiosyncrasies of each individual appliance. PA1 6) A DCM Manager--Installs the DCMs. It automatically reacts to changes in the network by installing new DCMs for new appliances. PA1 7) A Data Driven Interaction (DDI) Controller--renders a GUI (Graphical User Interface) on a appliance's display on behalf of a HAVi software element. It supports a wide range of displays, varying from graphical to text-only. PA1 8) A Stream Manager (SMGR)--creates connections and routes real-time AV streams between two or more appliances on the network.
The HAVi architecture specifies at least two levels of interoperability, referred to as level 1 and level 2.
Level 1 interoperability addresses the general need to allow existing devices to communicate at a basic level of functionality. To achieve this, level 1 interoperability defines and uses a generic set of control messages (commands) that enable one device to communicate with another device, and a set of event messages that it should reasonably expect from a device given its class (TV, VCR, DVD player, etc). To support this approach a basic set of mechanisms is required: device discovery; communication; and a HAVi message set.
As to device discovery: each device in the home network needs a well-defined method that allows it to advertise its capabilities to others. The HAVi approach is to utilize so-called SDD data: self describing data. The SDD data is required on all devices in the network. SDD data contains information about the device which can be accessed by other devices. The SDD data contains, as a minimum, enough information to allow instantiation of a so-called embedded device control module (embedded DCM). An embedded DCM is a piece of code pre-installed on a controlling IAV or FAV in platform-dependent code and using native interfaces to access the IAV's or FAV's resources. As mentioned above, a DCM for a device is a software element that provides an interface for control of general functions of the device. Instantiation of an embedded DCM results in registration of the device's capabilities with a registry. The registry provides a directory service and enables any object on the network to locate another object on the network. Registering allows applications to infer the basic set of command messages that can be sent to a specific device on the network.
As to communication: once an application has determined the capabilities of a device, the application needs to be able to access those capabilities. This requires a general communication facility allowing applications to issue requests to devices. This service is provided by the HAVi messaging systems and DCMs. The application sends HAVi messages to DCMs, the DCMs then engage in proprietary communication with the devices.
As to HAVi message sets: in order to support level 1 interoperability a well-defined set of messages is required that must be supported by all devices of a particular known class (e.g., the class of TV receivers, the class of VCR's, the class of DVD players, etc.). This ensures that a device can work with existing devices, as well as with future devices, irrespective of the manufacturer.
These three basic requirements support a certain minimal level of interoperability. Since any device can query the capabilities of another via the registry, any device can determine the message set supported by another device. Since applications have access to the messaging system, any device can interact with any other device.
Level 1 interoperability ensures that devices can interoperate at a basic level of functionality. However, a more extended mechanism is needed to also allow a device to communicate to other devices with any additional functionality that is not present in the embedded DCM's on an FAV. For example, embedded DCM's may not support all features of existing products and are unlikely to support those totally new ones of future product categories. Level 2 interoperability provides this mechanism. To achieve this, the HAVi Architecture allows uploadable DCM's as an alternative to the embedded DCM's mentioned above. The uploaded DCM may replace an existing DCM on an FAV. An uploadable DCM may be provided by any suitable source, but a likely technique is to place the uploadable DCM in the HAVi SDD data of the BAV device, and upload from the BAV to the FAV device when the BAV is connected to the home network. Because the HAVi Architecture is vendor-neutral, it is necessary that the uploaded DCM will work on a variety of FAV devices all with potentially different hardware architectures. To achieve this, uploaded DCMs are implemented in HAVi (Java) bytecode. The HAVi bytecode runtime environment on FAV devices supports the instantiation and execution of uploaded DCMs. Once created and running within a FAV device, the DCM communicates with the BAV devices in the same manner as described above.
The efficiency of level 2 interoperability becomes clear when one considers resources needed to access a specific device functionality. Level 2 allows a device to be controlled via an uploaded DCM that presents all the capabilities offered by the device, whereas to achieve similar functionality in level 1, this DCM would have to be embedded somewhere in the network. For example, when a new device is added to a network, level 1 requires that at least one other device comprises an embedded DCM compatible with the new device. In comparison, level 2 only requires that one device provide a runtime environment for the uploaded DCM obtained from the new device.
The concept of uploading and executing bytecode also provides the possibility for device-specific applications called Device Control Applications. Through these applications a device manufacturer can provide the user a way to control special features of a device without the need for standardizing all the features in HAVi. The application is provided by a DCM in HAVi bytecode and can be uploaded and installed by each FAV device on the network.
For further information, reference is made to the HAVi specification and the IEEE 1394 specification that are available in the public domain. The HAVi core specification has been made available on the web at, for example, http://www.sv.philips.com/news/press, and is incorporated herein by reference.